The present invention relates to improved reactors for catalytic gas-phase reactions and processes to be carried out therewith, in particular improved oxidation processes such as processes for the oxidation of ammonia which can be used, for example, as components in the preparation of caprolactam or in particular of nitric acid.
In carrying out catalytic gas-phase reactions, heat of reaction is evolved in many cases. The temperature increase caused thereby in the interior of the reactor can represent a hazard, in particular when explosive starting material mixtures are employed.
The heat evolved in exothermic gas-phase reactions can reach the sections of the reactor located upstream of the reaction zone, for example by thermal conduction via the heated reactor walls, by convection due to swirling reaction gases traveling in the countercurrent direction or as a result of heat radiation. As a consequence of this heat transport, the feed gas can be heated so strongly before reaching the reaction zone that undesirable secondary reactions or uncontrolled prereactions can occur before reaching this zone.
An example of an exothermic gas-phase reaction which is carried out industrially on a large scale is the preparation of nitric acid (“HNO3”). This is generally carried out on an industrial scale by catalytic oxidation of ammonia over Pt/Rh catalysts in the Ostwald process. Here, NH3 is oxidized very selectively to NO which is then oxidized to NO2 during the course of the further process and is finally reacted with water in an absorption tower to give HNO3. The Pt/Rh catalysts are configured as fine gauzes and are stretched over a wide area in a burner. Typical dimensions for these gauzes are diameters of 0.5-5 m. The thickness of the gauze packing is usually, depending on the number of gauzes used, from a few millimeters to a maximum of 2 centimeters. A gas mixture typically comprising about 8-12% by volume of ammonia and air is passed through the gauzes, with a temperature of about 850-950° C. being established at the gauzes due to the exothermic reaction.
The hot reaction gas is subsequently cooled in a heat exchanger in which steam is generated or process gas is heated.
The reason for the catalyst geometry chosen, viz. a large diameter and very small height of the gauzes, is that the oxidation of NH3 firstly has to occur at a very short residence time because of possible subsequent reaction of the NO and, secondly, the pressure drop caused by flow through the gauzes and mechanical stress on the gauzes have to be kept as low as possible. Thus, flow through the gauzes in industrial HNO3 production occurs at a relatively low linear velocity of, depending on the pressure range, about 0.4-1.0 m/s under atmospheric conditions, about 1-3 m/s in the case of intermediate-pressure combustion in the range 3-7 bar abs and about 2-4 m/s in the case of high-pressure combustion in the range 8-12 bar abs, with the velocities indicated being superficial velocities for the gas which has been heated by the heat of reaction. In addition, if the flow is too fast, the reaction on the Pt/Rh gauzes can be extinguished by the cooling action of the inflowing gas stream (“blow-out” phenomenon).
The lower limit for the inflow velocity of the ammonia/air mixture is marked by the flame velocity of possible thermal ammonia combustion, so that flashback of the reaction ignited on the catalyst into the free gas space upstream of the catalyst bed can be ruled out in any case.
Apart from the classical gauze catalysts, the use of base metal catalysts based on transition metal oxides for the oxidation of ammonia is described in the scientific and patent literature. These can be used either alone or in combination with Pt/Rh gauzes.
A review of this literature may be found, for example, in Sadykov at al., Appl. Catal. General A: 204 (2000) 59-87. The driving force for the use of base metal catalysts is the saving of noble metals, in particular platinum. Nobel metal catalysts are consumed in the oxidation of ammonia and therefore have to be replaced, depending on the throughput through the gauzes, at intervals of from about three months to one year, which incurs considerable costs.
The catalysts based on transition metal oxides are usually, like the Pt/Rh gauze catalysts, operated at relatively low inflow velocities. This is necessary, in particular, to avoid extinguishing the oxidation of ammonia again after it has been ignited on the catalyst. Catalysts based on transition metal oxides are generally less active than noble metal catalysts and compared to the latter have a significantly higher ignition temperature and also a higher extinguishing temperature.
WO-A-99/25,650 describes how the “blow-out” temperature can be decreased by the use of very finely particulate catalyst pellets accommodated in cartridges without the pressure drop being allowed to increase too much.
In the catalytic oxidation of ammonia, there is always the problem that ammonia can ignite before contact with the actual oxidation catalyst, e.g. on hot tube walls, and in this way be burned unselectively to N2 and H2O or N2O.
EP-A-1,028,089 states that back-radiation from ammonia combustion to distributor units for conveying the NH3/air mixture can lead to heating of these internals as a result of which part of the inflowing NH3 is oxidized to N2O on the surface of these internals.
The problem of NH3 preignition is of particular significance at the industrially relevant, high NH3 concentrations of 8-12% by volume, since here combustion is self-sustaining and can even be reinforced as a result of the heat evolved in the reaction.
In addition to the actual ignition temperature, i.e. the critical surface temperature above which NH3 decomposition can occur, the removal of the heat liberated by NH3 decomposition is therefore also of critical importance.
This removal is improved the faster the gas stream laden with ammonia flows over the surfaces (cooling action) and the colder this stream is. In addition, the residence time of the feed gas stream before contacting with the catalyst is shortened and the reaction time of the possible unselective prereaction is thus also shortened.
In the industrial preparation of HNO3 by oxidation of ammonia over Pt/Rh gauzes, the low initiation temperature of the highly active Pt/Rh catalysts makes a relatively low inlet temperature of about 200° C. possible. In this way, ammonia preignition is no obstacle to industrial implementation of the process despite the low inflow velocities.
However, when catalysts having a low catalytic activity are used, the feed gas mixture has to be at higher temperatures (preheating) or the process has to be operated at lower inflow velocities, or preferably a combination of the two measures has to be employed. Under these conditions, the risk of ammonia preignition is increased.
Experiments using honeycomb catalysts which, compared to platinum gauzes, have a lower cross section and a greater depth of the catalyst bed have now shown that the selectivity to the formation of the desired NOx is only very small at low inflow velocities of the feed gas mixture. The economics of such a process is therefore questionable. This effect could theoretically be compensated by increasing the inflow velocity of the feed gas mixture. However, increasing the inflow velocity is in practice subject to limitations since a disproportionate increase in the pressure drop occurs and, in addition, only incomplete combustion of the ammonia is achieved under some circumstances.
The same problems exist in principle in other industrially operated exothermic gas-phase reactions, e.g. oxidation reactions other than the oxidation of ammonia, epoxidations or free-radical halogenations of hydrocarbons.